Gosh, this sounds familiar! Have a similar problem with DD. Drives me bananas. Everyone deals with this kind of thing differently, but here's my advice for what it's worth.
I don't think it's worthwhile talking to the teacher (especially, if as you say, she's ineffective anyway). Nor do I think it's a good thing to speak to his mum. Basically, unless your daughter is being physically hurt, or a victim of systematic bullying then I really think it's between the kids. Stay out of playground politics.
But this is a great time to talk to your daughter about this, and about the need to stick up for yourself, and be more assertive. She needs to learn this anyway, and develop these social skills, otherwise every time there's a problem, she'll expect mummy to fix it. And she won't have confidence in her own strength of character and ability to swim through these turbulent social waters.
When talking to her, you might suggest some things to say. Practice with her saying "no" firmly but politely, and "no I don't want to do that right now, thanks". With the "I don't want to be your friend anymore" she might respond "well, that hurts my feelings" or "okay, I'll play with someone else, then". Give her some tools to deal with this herself.
Good advice from Hassled re: invite other children over/encourage other relationships.
BTW my daughter is finally distancing herself a bit from this boy. She actually said to him "I like you, but you're not the boss of me, you know". Hey, result!